Together with drastic increase of the demand for communication in recent years, development of a wavelength division multiplexing communication system (WDM communication system) for multiplexing a plurality of signal lights having wavelengths different from each other to implement high-capacity transmission with one optical fiber is advancing.
In such a wavelength division multiplexing communication system as just described, it is demanded for a semiconductor laser to oscillate with a wavelength suitable for a predetermined channel. Therefore, it is essentially required to monitor and control the  oscillation wavelength of a semiconductor laser which is a light source.
For example, there is a technique wherein a module section having a tunable light source device including a semiconductor laser is configured so as to have an etalon filter having a particular period transmission spectrum and an optical detector and the oscillation wavelength of the tunable light source device is monitored and controlled (hereinafter referred to as first technique).
Further, for example, there is a technique wherein a semiconductor laser device and an optical detector section (including a photodiode and an etalon filter) are provided as a semiconductor laser module in which a beam splitter need not be provided (hereinafter referred to as second technique).
Further, for example, there is a technique wherein the oscillation wavelength is controlled in a DBR type semiconductor laser in which a distribution reflector and an optical detector are integrated (hereinafter referred to as third technique).
Further, for example, there is a technique wherein the oscillation wavelength of a laser is monitored in a three-electrode DFB semiconductor laser in which a tunable semiconductor laser, a distribution reflector including a grating, an optical isolator and an optical receiver are integrated (hereinafter referred to as fourth technique). 